Is the central‐marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L

Abstract

The central‐marginal hypothesis (CMH) posits that range margins exhibit less genetic diversity and greater inter‐population genetic differentiation compared to range cores. CMH predictions are based on long‐held “abundant‐centre” assumptions of a decline in ecological conditions and abundances towards range margins. Although much empirical research has confirmed CMH, exceptions remain almost as common. We contend that mangroves provide a model system to test CMH that alleviates common confounding factors and may help clarify this lack of consensus. Here, we document changes in black mangrove (Avicennia germinans) population genetics with 12 nuclear microsatellite loci along three replicate coastlines in the United States (only two of three conform to underlying “abundant‐centre” assumptions). We then test an implicit prediction of CMH (reduced genetic diversity may constrain adaptation at range margins) by measuring functional traits of leaves associated with cold tolerance, the climatic factor that controls these mangrove distributional limits. CMH predictions were confirmed only along the coastlines that conform to “abundant‐centre” assumptions and, in contrast to theory, range margin A. germinans exhibited functional traits consistent with greater cold tolerance compared to range cores. These findings support previous accounts that CMH may not be a general rule across species and that reduced neutral genetic diversity at range margins may not be a constraint to shifts in functional trait variation along climatic gradients.

Effects of environmental heterogeneity on phenotypic variation of the endemic plant Lilium pomponium in the Maritime and Ligurian Alps

“…The lack of separation between geographically peripheral and central groups in traits related to pollination environment and the occurrence of an environmental marginal group near the geographical centre are in line with the idea that CHP predictions are confirmed only when its assumptions are met (Lira-Noriega and Manthey 2014 ; Kennedy et al 2020 ). Our results suggest that variability in local conditions drives variation in floral traits and probably in the pollination environment.…”

When ecological marginality is not geographically peripheral: exploring genetic predictions of the centre-periphery hypothesis in the endemic plantLilium pomponium

“…This inconsistence is probably because of the complexity of spatial, ecological, and/or historical factors across a geographical gradient ( Eckert, Samis & Lougheed, 2008 ; Pironon et al, 2015 ). As a result, although not all expectations are met, some predictions are often supported and the relative importance of the relationships among these factors may be dramatically different from one species to another, resulting in a lack of support for some CPH predictions, even those concerning reduced genetic variation within-population and enhanced differentiation among-populations that are among the most commonly observed trends ( Lira-Noriega & Manthey, 2014 ; Pironon et al, 2017 ; Kennedy et al, 2020 ). Several studies suggest that environmental gradients may be more important than geographical gradients for the expression of trends in genetic diversity ( Pilot et al, 2006 ; Cimmaruta, Bondanelli & Nascetti, 2005 ; Lira-Noriega & Manthey, 2014 ).…”

“…This pattern may be particularly important in shaping the spatial distribution of genetic variation in species growing in areas that have remained stable over time ( Nunes, Mancini & Bugoni, 2017 ), for example, in climate refugia ( Hewitt, 1999 ; Gavin et al, 2014 ; Hampe & Petit, 2005 ), regardless of their current size or density. Unfortunately, very few studies have been conducted on species historical ranges in relation to climate change ( Pouget et al, 2013 ; Kennedy et al, 2020 ).…”

Frequency of extreme freeze events controls the distribution and structure of black mangroves (Avicennia germinans) near their northern range limit in coastal Louisiana

“…Coastal wetland scientists working in eastern North America have long recognized that the frequency of extreme freeze events governs the northern range limits of mangrove forests in Texas, Louisiana and Florida (Kennedy, Preziosi, Rowntree, & Feller, 2020; Lloyd & Tracy, 1901; Lonard & Judd, 1991; Lugo & Patterson‐Zucca, 1977; Sherrod & McMillan, 1985; West, 1977). However, extreme freeze events that lead to mangrove mortality or damage are infrequent and difficult to study (Osland, Day, et al, 2020; Pickens, Sloey, & Hester, 2019; Ross et al., 2009).…”